This invention relates to an electromagnetic proportional pressure control valve capable of producing pressure proportional to electric current supplied thereto.
A conventional electromagnetic proportional pressure control valve of the general type comprises a housing having a guide hole therein. A primary port, a secondary port and a tank port (discharge port) all of which communicate with the guide hole are arranged in this order along the length of the guide hole. A spool is received in the guide hole of the housing for movement therealong. In accordance with the movement of the spool, the communication of the secondary port with the primary port and the discharge port is controlled. An electromagnetic actuator is mounted on the housing, and when electric current is supplied to this electromagnetic actuator, an electromagnetic force proportional to the magnitude of such electric current is applied to the spool. This electromagnetic force moves the spool in such a direction as to communicate the secondary port with the primary port and to interrupt the communication between the secondary port and the discharge port, that is, in a direction to increase a secondary pressure in the secondary port.
The spool has land portions disposed respectively on the opposite sides of the secondary port, and because of the difference between the pressure receiving areas of these land portions, a force due to the pressure of the secondary port is applied to the spool. This force moves the spool in such a direction as to communicate the secondary port with the tank port and to interrupt the communication of the secondary port with the primary port, that is, in a direction to decrease the secondary pressure.
In this conventional electromagnetic proportional pressure control valve, the secondary pressure is so controlled that the electromagnetic force can be balanced with the force due to the secondary pressure. As a result, the secondary pressure is substantially proportional to the value of the electric current supplied to the electromagnetic actuator. In this conventional control valve, the effective pressure receiving area of the spool for receiving the primary pressure (pump pressure) in the primary port is zero, and the force due to the primary pressure is not applied to the spool. Therefore, in this conventional control valve, the secondary pressure is determined only by the value of the electric current supplied to the electromagnetic actuator, and even when the primary pressure varies, the secondary pressure is not influenced by such variation in the primary pressure. Although the spool is provided with a spring urging the spool in a direction opposite to the direction of the electromagnetic force, this spring is intended to restrain vibrations of the spool when the electromagnetic force is zero, and the force of this spring can be disregarded during the above control of the secondary pressure.
Other known electromagnetic proportional pressure control valves, as disclosed in Japanese Laid-Open (Kokai) Patent Application Nos. 98409/87 and 110085/87 and U.S. Pat. No. 4,763,872, utilize a pilot pressure. Such conventional control valve has a pilot chamber provided at one end of a spool, the pilot chamber communicating with a primary port via an orifice formed in the spool. The pilot chamber also communicates with a tank via a valve port formed in the housing. This valve ports closed and opened by a pilot valve. The pilot valve is subjected to an electromagnetic force of an electromagnetic actuator to be urged in its closing direction. The position of the pilot valve, that is, the degree of opening of the valve port, is so controlled that the force due to the pilot pressure in the pilot chamber can be balanced with the force of the electromagnetic actuator. With this arrangement, the pilot pressure is proportional to the value of the electric current supplied to the electromagnetic actuator. On the other hand, the pilot pressure is applied to the end of the spool, so that the spool is urged in a direction to increase the secondary pressure, that is, in a direction opposite to the direction of the force due to the secondary pressure.
In this conventional control valve, the fluid is fed from the primary port to the pilot chamber. However, the pilot pressure is lower than the primary pressure because the fluid flows through the orifice, and irrespective of the primary pressure, the pilot pressure is determined only by the value of the electric current supplied to the electromagnetic actuator. Therefore, in this conventional control valve, also, the secondary pressure is determined only by the value of the electric current supplied to the electromagnetic actuator, and even when the primary pressure varies, the secondary pressure is not influenced by such variation in the primary pressure.
In this application of the above control valve to certain uses, it is required to utilize not the secondary pressure alone but the difference between the primary pressure and the secondary pressure. Moreover, it is sometimes required to utilize such pressure difference accurately proportional to the value of the electric current. However, the conventional electromagnetic proportional pressure control valves have the disadvantage that the difference between the primary pressure and the secondary pressure is influenced by variations in the primary pressure. The reason for this is that even when the primary pressure varies, the secondary pressure is not influenced by such variation in the primary pressure.
In an electromagnetic proportional pressure control valve disclosed in Japanese Laid-Open Patent Application No. 261782/87, the force due to the secondary pressure and the force due to the pilot pressure are both applied to a spool in the same direction to decrease the secondary pressure, and the force of a spring acting against these two forces is applied to the spool in a direction to increase the secondary pressure. In this conventional control valve, the force of the spring is an important factor for the pressure control. However, even in this conventional control valve, the secondary pressure is not influenced by variations in the primary pressure and is controlled in proportion only to the value of the supply electric current, as in the above-mentioned conventional control valves.
U.S. Pat. No. 4,860,792 filed by the Applicant of the present application discloses an electromagnetic proportional pressure control valve having passages formed in both a spool and a housing.